Gravitational Wave Signals from Domain Walls as Tests of the Seesaw Mechanism

Neutrinos are the least understood particles of the Standard Model. The mechanism behind their nonzero masses is unknown and requires postulating the existence of new particles and interactions. Given that the measured neutrino masses are very small, the physics behind their generation is naturally set at a high scale (via the seesaw mechanism), very difficult to probe in conventional particle physics experiments. Surprisingly, this is perfectly suited for being tested in gravitational wave experiments via early Universe domain wall and cosmic string production, as well as first order phase transitions. The simplest UV complete realization of the seesaw mechanism is via a U(1) gauge symmetry broken at a high scale. For concreteness, I am going to consider the case when this U(1) symmetry is either lepton number or baryon minus lepton number (B-L), accompanied by an additional U(1) symmetry, e.g., baryon number, also broken at the high scale. If any of the symmetry breaking proceeds via more than one scalar, this may lead to the production of domain walls through an intermediate Z2 discrete symmetry breaking. One therefore expects a new type of signal involving a double-peaked domain wall spectrum, or a domain wall signal overlaying a cosmic string spectrum. This novel type of gravitational wave signature can be searched for in upcoming experiments like Cosmic Explorer, Einstein Telescope, DECIGO, Big Bang Observer and LISA.